I read in a biology course that you can determine the concentration of oxygen in water by adding some NaOH and then a small amount of MgClX2. They say that a dark coloured precipitate is formed. I can't see a way of writing this reaction. Can anyone help me?
Answer
This is the old Winkler titration, published as
Die Bestimmung des im Wasser gelösten Sauerstoffes by Ludwig Wilhelm Winkler in Ber. Dtsch. Chem. Ges., 1888, 21, 2843-2854 (SRC)
How does it work?
- Add MnSOX4 to your water sample, then NaOH and KI.
Under alkaline conditions, oxygen dissolved in your water sample oxidizes Mn(II) to Mn(IV). 2MnX2++OX2+4OHX−⟶MnO(OH)X2↓ This is the brownish precipitate mentioned in the question.
- Now, the sample is acidified (Winkler used hydrochloric acid) and the solution becomes yellowish due to the iodine formed. Under these conditions, Mn(IV) oxidizes iodide:
Mn(IV)+2IX−⟶MnX2++IX2
- The amount of iodine formed, which repesents the aount of oxygen in the water sample, can be determined by titration with thiosulfate solution.
No comments:
Post a Comment